Businesses and consumers use a wide array of high-frequency transmitters and receivers. These transmitter and receivers are used in unidirectional and bi-directional wireless devices (e.g., cell phones, wireless LAN cards, GPS devices), as well as unidirectional and bi-directional wireline devices (e.g., network interface cards). Increasingly, manufacturers of high-frequency communication devices are integrating most or all of the transmitter and receiver (generally, transceiver) circuitry onto a single integrated circuit die (i.e., chip) or at least onto a very small number of chips. As has been well reported, a number of technical obstacles have been encountered on the road to development of single chip transceivers.
One of these technical obstacles has been the tuning range, tuning accuracy, and phase noise of oscillator components in the transmitters and/or receivers. The core of many oscillators comprises some type of resonator device and associated tuning circuitry. The resonators include surface acoustic wave (SAW) resonators, microelectromechanical (MEM) resonators, and film bulk acoustic wave resonator (FBAR) devices.
FBAR devices are highly advantageous for use in oscillator applications because: 1) their high electro-acoustic coupling allows maximum frequency pulling, 2) their small size offers economic wafer fabrication and compact hybrid integrations and assembly; and 3) their planar process technology is most compatible with CMOS fabrication. Conventional varactor-tuned L/C circuits are less suitable because the resonant frequency and Q are defined by inaccurate and lossy inductor (L) and capacitor (C) components. FBAR devices are particularly useful for wireless transceiver applications.
Electro-acoustic RF resonators, as realized in film bulk acoustic wave resonator (FBAR) technology, exhibit a series resonance tightly followed by parallel resonance. Conventional techniques for tuning either resonance with a series or parallel variable capacitor, respectively, are limited to the spacing of the two resonances. Even an ideal varactor with a capacitance range from 0 picofarads (pF) to ∞ (infinite) pF cannot exceed these limits. A varactor with a typical Cmax/Cmin of 2.5 is able to pull either resonance frequency only a small fraction of the frequency spacing between F(parallel) and F(series). The foregoing limits the usefulness of FBAR devices in single chip transceiver applications.
Therefore, there is a need in the art for improved transmitters and receivers for use in high-frequency communication devices. In particular, there is a need in the art for high-frequency oscillators that are stable and accurate, while having a wide frequency tuning range. More particularly, there is a need for FBAR devices having a wide frequency tuning range for use in high-frequency transceiver oscillators.